1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same. More specifically, the present invention relates to an interlayer insulating film formed between a lower level wiring conductor layer formed of a refractory conductor material and an upper level wiring conductor formed of an aluminum type metal film.
2. Description of Related Art
In a conventional semiconductor device, such as a MOS semiconductor device, having an upper level wiring conductor formed of an aluminum type metal film, an interlayer insulating film formed of a BPSG (borophosphosilicate glass) film has been widely used. For example, this MOS semiconductor device has been essentially manufactured as follows: A semiconductor device having a diffusion layer and a polysilicon wiring conductor at a silicon substrate surface is formed, and a non-doped insulating film is formed on a whole surface of the substrate. Thereafter, a BPSG film is formed on a whole surface of the non-doped insulating film. The non-doped insulating film is provided for the purpose of preventing phosphorus or boron contained in the BPSG film from being diffused into the diffused layer. The BPSG film is reflowed by a heat-treatment for example in a nitrogen or steam atmosphere at a temperature on the order of 900.degree. C., so that the surface is smoothed. In order to reflow the BPSG film, a temperature of not less than 850.degree. C. is required. An opening is formed through the BPSG film and the non-doped insulating film, and an upper level wiring conductor composed of an aluminum type metal film is formed. A main mason for adopting the BPSG film as the interlayer insulating film lies on a gettering property and on a reflow property.
With advanced microminiaturization of semiconductor devices, a problem occurs in connection with the reflow of the BPSG film in the case that the interlayer insulating film is formed of only the BPSG film.
Now, referring to FIGS. 1A and 1B which are sectional views of a semiconductor device, and to FIG. 2 which is a graph showing a step coverage of the BPSG film, explanation will be made on the fact that when a BPSG film is deposited, a void occurs dependently upon the step coverage of the BPSG film.
Firstly, as shown in FIG. 1A, a silicon oxide film 211A is formed on a P-type silicon substrate 201A, and a polysilicon gate electrode 206A is formed on the silicon oxide film 211A. Then, a BPSG film 220 of a thickness "b" is formed on the whole surface. This case will be now examined. The concentration of each of phosphorus and boron contained in this BPSG film 220 is 5 mol %. Here, it is assumed that a minimum thickness of the BPSG film formed on a side surface of the polysilicon gate electrode 206A is "a". In this case, a factor "a/b" is a parameter showing the step coverage, and the dependency of the parameter "a/b" upon the thickness "b" of the BPSG film 220 is as shown in FIG. 2. Namely, when the thickness "b" of the BPSG film 220 is not greater than 200 nm, the step coverage of the as-deposited film is excellent, but when the thickness "b" of the BPSG film 220 is greater than 200 nm, the step coverage of the as-deposited film degrades accordingly. However, if the thickness "b" of the BPSG film 220 is not greater than 200 nm, a stray capacitance which formed between the polysilicon gate electrode and the upper level wiring conductor composed of aluminum type metal film becomes unfavorably large.
Next, the case that the thickness "b" of the BPSG film 220 is larger than 200 nm, will be examined. As shown in FIG. 1B, a silicon oxide film 211B is formed on a P-type silicon substrate 201B, and a plurality of polysilicon gate electrodes 206B are formed on the silicon oxide film 211B with intervals of 0.5 .mu.m. Then, a BPSG film 221 having a thickness of for example 250 nm is formed on the whole surface. The concentrations of phosphorus and boron contained in the BPSG film 221 are the same as those of the phosphorus and the boron contained in the BPSG film 220, respectively. In this case, if the step coverage is excellent, the surface of the as-deposited BPSG film 221 should be substantially planar. Actually, however, voids 215 occur in the as-deposited BPSG film 211, as will be readily understood from the result shown in FIG. 2.
These void 215 will disappear when the BPSG film 211 is reflowed. For this reflow treatment, a temperature of not less than 850.degree. C. is required as mentioned hereinbefore. However, if the heat-treatment is performed at a temperature of not less than 800.degree. C., a depth of junction will increase in source/drain regions of a MOS transistor, with the result that a so-called short channel effect becomes large. This phenomenon is remarkable, particularly in a P-channel MOS transistor. Because of this reason, the interlayer insulating film formed of the reflowed BPSG film is not suitable in a semiconductor integrated circuit including microminiaturized semiconductor devices.
Recently, as an insulating film having an excellent step coverage, attention is focused on a non-doped silicon oxide film formed by a chemical vapor deposition process using ozone (O.sub.3) and tetraethoxysilane (Si(OC.sub.2 H.sub.5).sub.4, called "TEOS" hereinafter). This non-doped silicon oxide film will be called an "ozone-TEOS NSG film" hereinafter. Here, "NSG" is an abbreviation of a "non-doped silicate glass".
However, if the interlayer insulating film is formed of only the ozone-TEOS NSG film, other problems will occur. One of the other problems is that the ozone-TEOS NSG film itself has no gettering function. In addition, the as-deposited ozone-TEOS NSG film has a high moisture content, and therefore, a heat-treatment is required. During the heat-treatment, however, the moisture moves to the semiconductor devices. Therefore, if the interlayer insulating film is formed of only the ozone-TEOS NSG film, an electrical characteristics of the semiconductor devices is deteriorated.